Pharmacologic doses (10 to 240 mg, p.o) of the hormone melatonin, which is normally secreted by the pineal gland at night, have been shown to produce daytime fatigue and performance changes normally associated with sleepiness. Preliminary data from our laboratory suggest that much lower daytime melatonin doses, which elevate plasma melatonin levels within the normal nocturnal range, can also produce changes in temperature, mood, and performance similar to those produced by pharmacologic melatonin doses, and occurring nocturnally with sleep deprivation in otherwise untreated people. Two studies are proposed by a newer, less experienced investigator, to evaluate the effectiveness of sub-pharmacologic doses of melatonin as a hypnotic. Both studies will measure the dose-response relationships between melatonin dose, plasma melatonin levels, sleepiness, temperature, mood, and performance (using a battery of performance tasks previously shown to be sensitive to the effects of exogenous melatonin and sleep deprivation). Study I will test the effects of melatonin (0.1, 0.3, 1.0, or 10.0 mg, p.o.) or placebo, administered at 1145 h. Study II, designed to gather preliminary data for a more thorough study in the future, will examine the response to melatonin or placebo if ingested one hour prior to the subject's normal bed-time. Healthy men and women (including members of racial/ethnic minority groups) will be tested during both studies. Blood will be sampled and serum melatonin levels determined. The possible use of very low doses of melatonin as a clinically useful hypnotic will be examined.